By way of introduction and background, the use of a quartz element as a force sensing resonator is known. The quartz piezoelectric element in a force sensing resonator is typically a vibrating quartz member in a single or double beam arrangement where an applied force is placed on a vibrating beam. The force is a function of pressure so that pressure measurements can be obtained. The applied force, when changed as a function of pressure, changes the resonant frequency of the piezoelectric quartz element. The change in frequency can be correlated to the change in force caused by a change in pressure. These devices are characterized by use of a pressure conversion device such as a bourdon tube which produces a force causing a proportionately small increment of dimensional change in the quartz element for a large range of pressure change in the bourdon tube.
Both amorphous and crystalline quartz are often preferred as the sensor material for use in pressure measurement devices because quartz has a low coefficient of thermal expansion, low hysteresis and good repeatability characteristics. Quartz, however, is difficult to shape in small and precise dimensions in an elongated beam or a cylindrical elongated rod. Structurally, quartz is stronger in compression than in tension. A beam member constructed from quartz with a square or rectangular cross section is very limited in tensile strength before breaking (a low tensile strength). In compression, a quartz beam member suitable for resonating or vibrating purposes will buckle if placed under significant compression forces. A quartz beam member designed for force measurement purposes usually has dimensions which make it relatively fragile and thus presents difficult manufacturing and assembly problems.
One type of force measuring device which utilizes a vibrating quartz resonator or quartz beam can be likened to two tuning forks coupled tine to tine and placed under tension or compression. This device has a high mechanical "Q". "Q" is the relationship of energy stored to energy lost and represents efficiency. As a practical matter, the dimensions of the resonator beam members which approximate the two tines of such a tuning fork will not exactly match dimensionally. Thus, the respective tines of the resonator beam members inherently have different resonant frequencies which lowers the overall "Q". It is also not practically possible to apply exactly equal forces (to be measured) to each tine or beam member which causes an error because of the differing resonant frequencies of each tine which, in turn, lowers the overall "Q".
In a single beam type of quartz resonator where the beam member is placed under compression for force measurement, the beam is subject to buckling and force moments. Under tension, a single beam member is limited to its breaking strength. A single beam is also very shock sensitive and delicate to handle. While single beam quartz resonators overcome the drawbacks of tuning fork resonators, they must be decoupled from reactions with the mounting structure to obtain a high "Q".
By far the most disadvantageous characteristics of quartz beams is the difficulty of production, the fragile nature of quartz in small dimensions and low tension limitations.